UPDATE: Since this story was first published, Michael Vilkin went on trial for the fatal shooting of John Upton. A jury found Vilkin guilty of first-degree murder and assault with a deadly weapon. Vilkin will return to court in August for sentencing. He faces 50 years to life in prison.

In the tale of two neighbors from the wealthy community of Encinitas in San Diego, Calif., with a tumultuous relationship that ended with one's death, their friends and loved ones continue struggling to make sense of the tragedy.

John Upton, 56, was a well-known do-gooder and filmmaker who devoted his life to helping children and the elderly.

"I really knew from a very, very young age that you really can make a difference because I saw my dad do it," Elizabeth Upton, John Upton's daughter, told ABC News' "20/20."

The other neighbor, Michael Vilkin, 61, was an intellectual inventor who emigrated from the former Soviet state of Georgia for the American dream.

On March 28, 2013, San Diego County sheriff's deputies responded to Vilkin's 911 phone call saying he killed his neighbor. Upon arriving to Vilkin's property, they found Upton lying on the ground, and he was later declared dead at the scene.

Vilkin was charged with premeditated murder and assault with a deadly weapon. He faces 25 years to life in prison if convicted, although he's confident that won't be the case.

"That cannot happen, because I did not assault him. He assaulted me," Vilkin said in a jailhouse interview with ABC News affiliate KGTV. "So how come that I will be found guilty? That is not possible."

According to other neighbors and Vilkin's wife, a narrow strip of land in front of Upton's front door on Vilkin's property, which Vilkin bought for his wife Tamara as a gift, was a point of contention between Upton and Vilkin.

"That guy John was obsessed about privacy, and he did not want me to cut the bushes or trees there for more than a year," Vilkin said.

Tamara Vilkin explained her husband's work on their land.

"In order to do this survey, he needed to cut some bushes," she said, "so he called the police to make sure that everything will go smoothly."

Upton's girlfriend, Evelyn Zeller, told "20/20" that Upton never complained about his neighbor.

"He used to say many times, 'Wow, I admire his work ethic,'" she said.

Zeller said she and Upton were peaceful, and loved Buddhas, serenity and inspiration.

Upton not only inspired his children and girlfriend, but also many others through his personal mission of rescuing Romanian orphans. ABC News' "20/20" even featured Upton's mission in an episode during the 1990s.

However, other neighbors told "20/20" off camera that Upton would taunt Vilkin from his balcony regularly to the point that it changed Vilkin.

"He just said that: He wanted to have a gun," Tamara Vilkin recalled of Vilkin's decision to buy a .44 magnum before Vilkin and Upton's final confrontation.

In Zeller's version of events, Upton went over to Vilkin's property to declare a truce. She said the couple was preparing to move away the very next day.

"We had found our dream home," Zeller said. "'We're outta here,' that's what he went out to say."

Outside, Vilkin admitted he was hiding behind a bush with a gun lodged in his waistband. As Upton approached, he met his fate.

"I called for John," Zeller said. "I did not get a response ... so I look up the path and I see John lying on the path."

Zeller said she heard Vilkin say, "Don't get any [expletive] closer," and alleged that he then pointed the gun at her.

Vilkin said Upton threatened him first, with a gun.

"He walked to me, accusing, f-word, f-word, f-word, and pulled the gun," Vilkin told KGTV from jail. "I pulled out my gun, and I was ready for it, and I shot him. It was self-defense."

According to investigators, Upton may have pulled out a cell phone, but there was no other gun found at the scene.

Next year, it will be a jury's job to decide whether Vilkin is an obsessed landowner who snapped and deliberately killed his own neighbor or if he was a terrorized intellectual, driven to defend himself against repeated verbal threats.

Meanwhile, Upton and Vilkin's families are both still feeling the effects of what transpired between the two neighbors.

"Who is this guy," Upton's daughter, Elizabeth Upton, asked, "this man who murdered my father in cold blood? For what?"

Upton's wide-reaching charity work inspired his children to follow in their father's footsteps. Both Johnny and Elizabeth Upton have traveled extensively to help orphans abroad.

Elizabeth Upton makes documentaries for a non-profit group her father started called Media4aCause, which provides short documentaries and brand training to worthy causes and charities for free, and Johnny Upton works in Uganda. Both said their work is a way to feel closer to their father.

Tamara Vilkin wishes he never bought the land for her "because it brought nothing but pain to us."

Tune in to see the full story on ABC News' "20/20" on Friday, Sept. 20, 2013, at 10 p.m. ET.